Elevated temperatures inside internal combustion engines can produce nitrogen oxides in exhaust gases. Regulations in many jurisdictions limit the emission of nitrogen oxides into the atmosphere because of their known adverse effects on the environment. One way of controlling nitrogen oxide emissions is injection of a hydrogen rich chemical into the exhaust stream, such as urea in an aqueous solution. The urea, or similar chemical, undergoes a chemical reaction that converts nitrogen oxides within the exhaust fumes into harmless nitrogen and water. Such systems can significantly reduce the concentration of nitrogen oxides in the exhaust fumes to comply with environmental regulations.
The urea or other hydrogen rich material that is dosed into the exhaust stream must be filtered in order to prevent damage to the dosing module. However, unlike most fluid filters necessary for operation of internal combustion engines, the hydrogen rich material necessary for removal of nitrogen oxides is typically an aqueous solution, and as such is susceptible to freezing at low temperatures. When an aqueous solution freezes, it expands in volume. The increase in volume at the fluid freezing temperature is dependent on the type of aqueous solution, but is often in the range of ten percent. Expansion in volume can lead to significant increases in pressure if that expansion occurs within an enclosed vessel, such as a filter housing. When volume expansion is not accounted for, freezing of the fluid can increase pressure inside the filter housing and result in damage to the housing, filter element or both.
Freezing patterns in fluids follow thermal patterns and vary significantly depending upon the geometry of the vessel containing the fluid. A liquid starts to solidify at the lowest temperature at or below the freezing temperature. This phenomena can start anywhere inside a filter housing. Temperature of a fluid inside a housing is usually not constant because there can be thermal patterns and temperature gradients. As the overall temperature drops, an aqueous solution starts to freeze at the lowest temperature location. This location can occur anywhere in the fluid and can entrap a liquid in a section of the housing. If that entrapped liquid is unable to find a volume into which it can expand, the filter housing and elements can be damaged.
One approach to solving the problem of liquids freezing in a filter housing is the use of a flexible or elastic filter housing, such as that described in U.S. Pat. No. 4,842,737. A significant drawback to such designs is that the use of a deformable filter housing imposes serious constraints on how the filter housing is mounted on an engine, because additional space needs to be available for the filter housing to expand during freezing of the fluid inside the filter. Another drawback with flexible housings is that it is difficult to construct a flexible filter housing that matches the durability and strength of a rigid filter housing. Thus, flexible housings increase the risk for mechanical failure and possible leaks, and are disfavored over rigid housings.
Another available approach to addressing the problem of liquids freezing within a filter housing is described in Japanese Patent No. JP61197013-A, which uses a flexible membrane attached to the inside of the housing of the filter and which delimits a certain volume of air on one side of the membrane. When the liquid in the filter freezes and expands in volume the air pocket behind the membrane is designed to take up the increased volume of the liquid while deforming the membrane elastically. However, this membrane arrangement is rather complex and fragile, which increases the risk for leakages and mechanical failure if the membrane fails or a leak otherwise develops.
A similar approach incorporates a compressible foam component instead of a flexible membrane. Such component generally has a multitude of air-containing cells that at least partially collapse when exposed to liquid pressure exceeding one bar of absolute pressure. Over time, repeated pressure changes resulting in repeated expansion and contraction of cell walls cause degradation of the foam component. In addition, the foam component, as it ages, can develop cracks that get penetrated by liquid, which can freeze and then thaw. Such changes in state also degrade cell walls of the foam component. Finally, exposure to petroleum also degrades many types of foam that can be implemented in these systems. Any degradation in the foam prevents the component from effectively absorbing volume when the aqueous solution freezes.
A similar approach to addressing the fluid-expansion problem is detailed in U.S. Pat. No. 7,481,319, which describes use of an expansion element placed in contact with the filter element inside a filter housing. These expansion elements also consist of a deformable foam material having air-containing cells that risk collapse when exposed to pressurized liquid. However, the expansion element can only take up small volume changes of liquid because it is made out of a deformable foam that has a high solids level. Also, the deformable element is in contact with the filter element, so the filter element cannot readily take up volume changes of the liquid in that direction. This limits the number of possible freezing patterns that can occur without damaging the filter or housing, and increasing the risk of damage to the filter housing, the filter element, or both. Lastly, this approach to addressing fluid-expansion problems is also susceptible to foam degradation for the reasons explained in the paragraph above.
Thus, a need exists for an improved filter construction for use in conditions where the fluid to be filtered is pressurized and subject to freezing.